One of two South Korean men detained in North Korea on charges of espionage is a pastor, a Seoul-based Protestant association said Monday.

Last week, the communist country said it is holding two South Korean men it identified as Kim Kuk Gi and Choe Chun Gil on charges of espionage. South Korea’s Unification Ministry confirmed that they are South Korean citizens, but did not provide details.

The General Assembly of Presbyterian Church in South Korea said Kim Kuk Gi is a pastor registered with the group. The group also called for his immediate release.

“He was carrying out missionary works for North Korean defectors in [the Chinese port city of] Dandong. He bought noodle makers and tofu machines to send to the North, as well as sewing machines,” an associate with the assembly said in a phone interview with the VOA Korean service.

The source, who preferred to remain anonymous, said he lost touch with Kim about two years ago.

This would not be the first time North Korea has detained a Christian missionary. American Kenneth Bae was believed to have been doing missionary work when he was detained by Pyongyang. He was freed last year after more than two years in captivity.

A Korean Canadian pastor is believed to be still being held by North Korea, but Pyongyang has not confirmed his status.

North Korea’s official media showed images of Kim Kuk Gi and Choe Chun Gil speaking at a news conference. The North Korean media said the two men were South Korean nationals working as spies for South Korea’s National Intelligence.

South Korea’s Unification Ministry denied the charges and urged the North to release them immediately. Seoul tried to deliver a message with a request for their release, but Pyongyang refused to accept it.

On Monday, South Korean Unification Ministry spokesman Lim Byeong-cheol said his government not only will continue to communicate with the North, but also closely coordinate with international organizations for the detainees’ release.

“By working with international organizations, we will raise awareness around the world that the North’s holding of our citizens against their will is a crime against humanity. We can put pressure on Pyongyang through various channels,” Lim said.

However, Lim said the government is not considering sending an envoy to Pyongyang to seek the release of the detainees. Lim added it will not be appropriate for the government to offer Pyongyang financial compensation for their release.

Another South Korean citizen has been held in the North since October 2013, making the total number of South Koreans being held captive in the country three.

Jee Abbey Lee contributed to this report, which was produced in collaboration with the VOA Korean service.